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Dust
from Africa Leads to Large Toxic Algae Blooms in Gulf of Mexico,
Study Finds
(Images below story)
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| This
image is of a large dust cloud leaving the West coast
of Africa on February 26, 2000. |
Saharan
dust clouds travel thousands of miles and fertilize the water
off the West Florida coast with iron, which kicks off blooms
of toxic algae, according to a new study. The research was
partially funded by a NASA grant as part of ECOHAB: Florida
(Ecology and Oceanography of Harmful Algal Blooms), a multi-disciplinary
research project designed to study harmful algae.
Toxic
algal blooms, sometimes called red tides, have in the past
killed huge numbers of fish, shellfish, marine mammals, birds,
and can cause skin and respiratory problems in humans.
Each
year iron from Saharan dust clouds is deposited in the waters
off the West Florida coast. Once there, plant-like bacteria
use the iron to set the stage for red tides. When iron levels
go up, this bacteria, called Trichodesmium, 'fixes' nitrogen
in the water, converting it to a form usable by other marine
life. The addition of biologically usable nitrogen in the
water makes the Gulf of Mexico a more likely environment for
toxic algae to bloom.
"This
is one of the first studies that quantitatively measured iron
from the dust and tied it to red tides through Trichodesmium,"
said Jason Lenes, a graduate student at University of South
Florida's College of Marine Science, and the lead author in
the study. Lenes works under John J. Walsh, one of the principal
investigators for ECOHAB, and one of the paper's coauthors.
The research appears in the September issue of the scientific
journal, Limnology and Oceanography.
Storm
activity in the Sahara Desert region generates clouds of dust
that originate from fine particles in the arid topsoil. Easterly
trade winds carry the dust across the Atlantic Ocean and into
the Gulf of Mexico. "Because iron is one of the most
common elements in most soils, a certain percentage of the
dust contains iron," said Lenes.
The
study used satellite and ground based measurements to track
large dust clouds leaving Africa on June 17, 1999. Lenes and
his colleagues followed the clouds using data from the Advanced
Very-High-Resolution Radiometer (AVHRR), an imager aboard
the National Oceanic and Atmospheric Administration's (NOAA's)
Polar Orbiting Environmental Satellites (POES).
The
Saharan dust reached the West Florida shelf around July 1st,
increasing iron concentrations in the surface waters by 300
percent. As a result, Trichodesmium counts shot up 10 times
what they had been prior to this event. Through a complex
process involving a special enzyme called nitrogenase, the
Trichodesmium used the iron to convert nitrogen in the water
to a form more usable for other marine life. In October, after
a 300 percent increase of dissolved organic nitrogen, a huge
bloom of toxic red algae (Karenia brevis) had formed within
the study area, an 8,100 square mile region between Tampa
Bay and Fort Myers, Florida.
Scientists
have labored for several years in an effort to develop a reliable
method to predict red tides, particularly because the results
of these blooms can be both physically and economically devastating
to a region.
"The
West Florida shelf is a hot spot for fishing, aquaculture
and tourism, all of which can be drastically affected by a
surprise visit from a red tide," said Lenes.
Humans
who swim in the Gulf can experience respiratory problems by
breathing toxins from K. brevis that get in the air. Also,
eating shellfish poisoned by red tides can lead to paralysis
and memory problems. Around the Gulf of Mexico, scientists
and others have recorded fish kills totaling in the millions
and manatee deaths in the hundreds resulting from a single
red tide bloom.
By
using satellites to monitor dust arrivals and Trichodesmium
blooms, Lenes said this research could lead to forecasting
of red tides. "If you could predict when a red tide is
coming, you could close beaches and fisheries ahead of time,"
Lenes said.
The
paper was funded through a number of grants connected with
the ECOHAB program. Other funding for this study included
grants from the National Oceanic and Atmospheric Administration,
the Office of Naval Research and the Environmental Protection
Agency. The ECOHAB: Florida Mission is to better understand
the factors involved in the occurrence and dispersal of Florida's
predominant red tide algae, K. brevis, and to predict and
manage harmful algal bloom events.
Satellite
Images of Saharan Dust Moving Across Atlantic, June and July,
1999
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Credit: NOAA Satellite Active Archive
These
two images above (click on each image to enlarge)
from a National Oceanic and Atmospheric Administration (NOAA)
satellite show iron-containing dust clouds blowing from the
African continent across the Atlantic Ocean in the months
of June and July, and reaching the eastern Gulf of Mexico
and west coast of Florida in July. The iron in the dust is
the key ingredient that sets off an environmental chain reaction
in the Gulf of Mexico waters that could lead to a Red Tide.
Every
year, during northern Africa's dry season, storm activity
in the Saharan Desert region generates clouds of iron-rich
dust from fine particles of the arid topsoil. The dust may
be carried more than 10,000 feet high into the atmosphere
by easterly trade winds.
The
trade winds then carry the dust over the Atlantic Ocean to
Florida. Typically, it takes one to two weeks for the Saharan
dust clouds to reach the continental United States.
The
study used satellite and ground based measurements to track
large dust clouds leaving Africa on June 17, 1999. The study's
lead author, Jason Lenes, and his colleagues, followed the
clouds using data from the Advanced Very-High-Resolution Radiometer
(AVHRR), an imager aboard the National Oceanic and Atmospheric
Administration's (NOAA's) Polar Orbiting Environmental Satellites
(POES).
The
second image depicts the Saharan dust as it reached the West
Florida shelf around July 1st, increasing iron concentrations
by 300 percent. As a result, Trichodesmium counts shot up
10 times what they had been before this one event. Trichodesmium
is a plant-like bacteria that utilizes iron in the dust to
bring nitrogen into the ocean water, a component required
for the red tide algae to bloom.
By
October, a huge bloom of Karenia breve, (a red tide), developed
within the research area, an 8,100 square mile area between
Tampa Bay and Fort Myers, Florida.
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Image
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| TOMS
dust and smoke (equatorial Africa) image from 17 June
1999. |
Satellite
Images of Saharan Dust Moving Across Atlantic, June 17 and
July 2, 1999
While
the previous NOAA images show aerosols blowing across the
ocean, these two images from NASA's Total Ozone Mapping Spectrometer
(TOMS) instrument show dust coming off regional land sources
in Africa as they follow their path across the Atlantic. The
TOMS instrument aboard the Earthprobe TOMS satellite, captured
these images of the dust event from June 17, 1999, as it leaves
Africa.
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Image
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| TOMS
dust and smoke (equatorial Africa) image from 2 July 1999. |
The
second image from July 2, 1999, shows the progression of this
event as it approaches North America. Credit: "Laboratory
for Atmospheres TOMS Project, NASA Goddard Space Flight Center"
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Image
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| This
image is of a large dust cloud leaving the West coast
of Africa on February 26, 2000. |
Image
of Saharan dust cloud leaving the west coast of Africa
Credit: February 26, 2000 SeaWiFS image.
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